[Dissertations 25 years after date 44. Behaviour at high temperatures of palladium based dental alloys]. / Proefschriften 25 jaar na dato 44. Hoogtemperatuurgedrag van palladium-basislegeringen.

In the Netherlands in the 1980s palladium based dental alloys were often used in substructures of metal-ceramic restorations. However, after their introduction to the market some unexplained problems came to light: porosity on the metal-ceramic interface, frequent failure of solder joints, margin lift and distortion of bigger constructions. Later, palladium allergies could be added to the list. A 1989 dissertation investigated these problems and underlying causes and came up with answers and solutions. Recent problems like the breakage and chipping of porcelain on zirconia cores shows again that industry is sometimes unaware of adjustment problems at the moment that a new product is introduced to the market.

[Clinical evaluation CAD/CAM crowns with a natural layering]. / Klinische evaluatie van CAD/CAM-kronen met een natuurlijke laagopbouw.

Around the year 2000, zirconia was introduced in dentistry. It is a material with great strength and attractive aesthetic properties, such as translucency and colour. Recent advances in digital dentistry are aimed at veneering the supporting structure ofzirconia with a layer of glass ceramics by means of an automated process. With the Primero process, a transparent outer layer of glass ceramic is applied directly onto the zirconia core, milled in the green stage and sintered. Because the restorations produced in this way have a two-layer structure like natural teeth, they obtain an aesthetic look similar to natural dental elements. The aim ofthis prospective study was to evaluate fit, marginal adaptation, contact with antagonists and neighbouring dental elements, aesthetics and colour. Clinical performance of Primero restorations were evaluated over a period of6 months and the results were compared with a previous study of CAD/CAM copings which were produced with the same method, but were veneered by hand in the traditional way.

Influence of thermal expansion mismatch and fatigue loading on phase changes in porcelain veneered Y-TZP zirconia discs.

There is a rapidly growing interest for the use of Y-TZP zirconia as core material in veneered all-ceramic prostheses. It was hypothesized that a mismatch in coefficient of thermal expansion between the veneering porcelain and the Y-TZP zirconia core of these prostheses causes transformation of the tetragonal to the monoclinic structure in Y-TZP zirconia at the interface boundary when exposed to fatigue loading, resulting in fracture at the interface boundary. Y-TZP zirconia discs were veneered with three porcelains differing in coefficient of thermal expansion. Finite element analysis was used to investigate the stress distribution in the bi-layered discs because of the mismatch in thermal expansion. Two of these three groups were fatigued with the veneering ceramic in tension. X-ray diffraction was used to measure the intensity of monoclinic and tetragonal zirconia phase present at the zirconia core surface after sintering, airborne abrasion, veneering, and fatigue loading. It was found that the sintered tetragonal structure was converted to monoclinic up to a depth of 27 mum after airborne abrasion, and reversed back to tetragonal after porcelain veneering with porcelain. Fatigue loading of veneered discs did not, even with the highest possible thermal mismatch stress, cause any conversion from tetragonal to monoclinic phase.

The CICERO system for CAD/CAM fabrication of full-ceramic crowns.

The CICERO method of crown fabrication consists of optically digitizing a gypsum die, designing the crown layer buildup, and subsequently pressing, sintering, and milling consecutive layers of a shaded high-strength alumina-based core material, a layer of dentin porcelain, and a final layer of incisal porcelain. Final finishing is performed in the dental laboratory. The CICERO method allows efficient production of all-ceramic restorations without compromising esthetics or function. This article reviews the process involved in the fabrication of a CICERO crown.

Computer modeling of occlusal surfaces of posterior teeth with the CICERO CAD/CAM system.

STATEMENT OF PROBLEM: Static and dynamic occlusal interference frequently needs to be corrected by selective grinding of the occlusal surface of conventional cast and ceramic-fused-to-metal restorations. CAD/CAM techniques allow control of the dimensional contours of these restorations. However, parameters responsible for the occlusal form need to be determined. In most articulators, these parameters are set as default values. Which technique is best for minimizing the introduction of occlusal interference in restorations has not been determined. PURPOSE: This study investigated differences in crown structure of a crown designed in static occlusion (STA) with designs adapted for dynamic occlusal interferences. Therefore, values from an optoelectronic registration system (String-Condylocomp, KAVO), an occlusal generated path (OGP) technique and default settings (DEF) were used in the CICERO CAD/CAM system. MATERIAL AND METHODS: Morphology of CON, DEF, and OGP crowns was compared with that of the STA crown with respect to differences in a buccolingual section and frequency of occlusal distances in an interocclusal range of 1 mm, measured from the occlusal surface of the crown. RESULTS: All crown types fulfilled the esthetic and morphologic criteria for restorations in clinical dentistry. Difference in the morphology of the OGP crown, compared with that of the STA crown, was greater than that for the CON and DEF crowns. These differences were seen especially in the distobuccal part of the occlusal surface; however, the number of occlusal contacts was considered sufficient to stabilize occlusion. CONCLUSION: Functional occlusion, adapted to dynamic occlusion in a CICERO crown for the first mandibular molar, can be obtained using data acquired with the String-Condylocomp registration system. The OGP technique was preferred to other techniques because of the simplicity of the technique for eliminating potential problems with opposing teeth during motion. However, this is achieved at the cost of fewer points of contact during occlusion than with the CON crown.

[CAD/CAM-copings for partial coverage]. / CAD/CAM-copings voor partiële omslijpingen.

Aim of the study was to evaluate the Computer Integrated Ceramic Reconstruction (CICERO)-system and the Ceramic Reconstruction (CEREC)-system for the production of all ceramic copings for partial coverage. Posterior teeth were prepared and the stone dies were made. Accuracy analyses were performed on ceramic restorations made by means of the CICERO and by means of the CEREC technique. The marginal gaps were compared to that of a control cast metal restoration. The results demonstrate that the marginal gaps of the CICERO and CEREC copings varied respectively for the premolar 58-80 microns (mean 69 microns) and 71-91 microns (mean 81 microns). For the upper molar 63-92 microns (mean 78 microns) and 68-110 microns (mean 89 microns) and for the lower molar 54-98 microns (mean 76 microns) and 73-99 microns (mean 86 microns). Control cast metal partial coverage restorations showed marginal gaps of 33, 49 and 41 microns. It is concluded that computers can produce copings for partial coverage preparations on stone dies with a mean marginal gap for CICERO copings of 74 microns and for CEREC copings of 85 microns. These values were obtained before optimizing the marginal fit by means of porcelain veneering.

Measurement of the margins of partial-coverage tooth preparations for CAD/CAM.

PURPOSE: This study tested the hypothesis that a scanning laser 3-dimensional digitizer is a precise and accurate instrument to measure chamfered and beveled margins of partial coverage tooth preparations for computer-aided design/computer-aided manufacturing (CAD/CAM). MATERIALS AND METHODS: The margins were measured by the digitizer on stone dies and calculated by triangulation into a 3-D representation. Instrument precision was defined as the ability to reproduce the same margin in repeated measurements and expressed as the coefficient of variation as a percentage. Instrument accuracy for chamfered and beveled margins was estimated by correlating their measurements to the measurement of the margin of a spherical calibration "phantom" with known dimensions. Accuracy was expressed as the standard deviation. RESULTS: The precision errors for the box- and cusp-chamfered margins and cusp-beveled margins were 3.9%, 3.4%, and 2.4%, respectively. With regard to accuracy the standard deviations of the measurements of the box- and cusp-chamfered margins and cusp-beveled margins were 19 microns, 21 microns, and 24 microns, respectively, compared to 15 microns for the phantom. CONCLUSION: Measurements of chamfered and beveled margins by a scanning laser 3-D digitizer for CAD/CAM are (1) precise (error < 4%) and (2) accurate, with a standard deviation of less than 9 microns compared to optimal measurements of the spherical margin of the phantom.

Ceramic-fused-to-metal restorations with a new CAD/CAM system.

Experience has shown that a cast restoration will rarely have proper occlusal contacts. One possible solution could involve the application of a computerized design and fabrication system for restorations with accurately calculated and computer-milled occlusal surfaces. The Cicero system for the production of ceramic-fused-to-metal restorations makes use of optical scanning, nearly net-shaped metal and ceramic sintering, and computer-aided crown fabrication techniques to fabricate restorations with maximal static and dynamic occlusal contact relations. The Cicero system produces crowns, fixed partial dentures, and inlays with different layers, such as metal and dentin and incisal porcelains, for maximal strength and esthetics. Fabrication of a crown for a mandibular first molar is described to demonstrate the computer-based design system.

Oxidation of a gold-palladium PFM alloy.

In this study, the influence of oxygen pressure, as well as oxidation time and temperature, on the oxide formation of a gold-palladium-indium-gallium alloy was investigated. After oxidation (vacuum/air, 1-10 min, 940-1020 degrees C) the oxides were removed selectively by means of leaching in 37% hydrochloric acid. Concentrations of the metallic parts of the oxide layers were determined by means of an emission spectrometer. The composition of the oxide layer was computed on the basis of these measurements. Palladium oxide was not found. The formation of In2O3 and Ga2O3 was detected. The oxidation time did not influence the composition of the oxide layer. A higher oxidation temperature gave a slightly greater In2O3 concentration. Oxidation in vacuum resulted in a much higher Ga2O3 content. Under all conditions gallium oxidized preferentially.

Carbon absorption of palladium-enriched ceramic alloys.

Palladium-enriched alloys are sensitive to carbon absorption. High carbon levels in ceramic alloys negatively influence some mechanical properties and cause porosities in the porcelain during the firing process. The influence of the palladium content on carbon absorption of alloys with a range of palladium content from 5 to 80% approximately was investigated. Carbon content was measured for alloys in the 'as received' condition as well as after melting once and three times. Carbon contents up to 200 ppm were measured in the case of a high-palladium system.

Corrosion of ferromagnetic alloys used for magnetic retention of overdentures.

Three ferromagnetic casting alloys intended for use with magnetically retained overdentures (compositions in wt%: #1, Pd 50, Co 47, Ga 2, Pt 1; #2, Pd 60, Co 37, Ga 2, Pt 2; #3, Pd 49, Co 46, Ga 2, Pt 2) were investigated. A 0.9% saline solution and an artificial saliva (Meyer) were used for evaluation of the electrochemical corrosion behavior of these alloys, utilizing standard potentiodynamic techniques. All alloys investigated possessed a good corrosion resistance in the potential range of the oral environment (from -100 to 300 mV versus SCE). In addition, alloys #1 and 3 appeared to be susceptible to pitting above +300 mV (SCE), while #2 was resistant to pitting and was found to passivate in 0.9% saline solution.